Forest trees and tropospheric ozone: role of canopy deposition and leaf uptake in developing exposure-response relationships

The development of ozone exposure-response relationships for forests must account for sources of variation in the atmosphere (e.g. ozone exposure dynamics) and the biosphere (i.e. ecological and physiological processes that govern pollutant transport and metabolic response). Whereas considerable effort has been devoted to atmospheric issues, the role of the biosphere is not well understood. A conceptual model is proposed to identify those ecological and physiological processes most relevant in developing exposure-response relationships for ozone, many of which are based on the uniqueness of ozone as an environmental stress. One of the greatest sources of variation underlying exposure-response relationships is the linkage between ozone transport from the troposphere through the stomata and intercellular spaces (gas phase) to metabolic sites of action in the leaf interior (liquid phase). The tropospheric pathway approaches meters in length, whereas the path length from the leaf surface to cellular target sites is less than a millimeter, and even though the latter segment is only a small fraction of the total pathway (less than 1%), physiochemical processes therein largely dictate the pollutant's fate and subsequent effects. Unfortunately, this segment of the pathway is not well characterized. Using a combination of process-level models operating at the levels of the atmospher-canopy interface and leaf surface/leaf interior, results are presented to indicate that ozone exposure dynamics in the leaf interior can be substantially uncoupled from that in the free troposphere and that ozone in the leaf interior is not homogeneously distributed. It is concluded that biological and chemical factors operating at the atmosphere/leaf interface are at least as important as physical exposure characteristics in the atmosphere in controlling exposure-response relationships for ozone.